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A lesion is any abnormality in the tissue of an organism (in layman's terms, "damage"), usually caused by disease or trauma. Lesion is derived from the Latin word laesio which means injury. Within psychology the term ablation or brain lesion is used with specific reference to experimentally induced damage or partial removal of animal brains for research purposes.

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DuVerney was the first to use experimental ablation method on animals in 1679. Flourens first published the method in 1824, describing the method and behavioral effect of brain damage.

Lesions done by knife cuts and suction techniques, called mechanical lesions, were tried by Veyssiere and Nothnagel in 1874. This process was done by inserting a fine wire blade through the head, rotating the curved or angled wire, and cutting neural projection. Baginski and Lehmann used this method with thin glass tube lowered through a small hole in the skull in 1886.

In 1895, Golsinger was the first to make electrolytic lesions in animals. In 1898 Sellier and Verger destroyed discrete areas in the caudate and anterior segment of internal capsule by passing current through double-needle insulated electrodes. This process kills neurons surrounding the electrodes.

In 1908, Horsley and Clark developed the stereotaxic method and combined it with electrolytic lesions to improve localization, precision, and reliability of brain damage in subcortical structures.

Lesions or ablation in psychology or an ablation experiment or lesion experiment is the removal of tissue to explore its function and was a basic research methodology in physiological psychology in the earlier part of the 20th century. The approach was refined in 1824 by Marie Jean Pierre Flourens, a French physiologist. It is used as a research tool in neurobiology. For example, by ablating specific brain regions and observing differences in animals subjected to behavioral tests, the functions of the removed areas may be inferred.

Studies show there is a correlation between lesion in the brain and language, speech, and category-specific disorders. However, lesions in Broca's and Wernicke's areas are not found to alter language comprehension.

Lesion in the amygdala would eliminate the enhanced activation seen in occipital and fusiform visual areas in response to fear with the area intact. Amygdala lesions change the functional pattern of activation to emotional stimuli in regions that are distant from the amygdala.

Lesion size is correlated with severity, recovery, and comprehension.

In the Wisconsin Card Sorting Test with unilateral frontal or nonfrontal lesions, patients with left frontal lesions did more poorly but had high perseverative error scores. In right frontal and nonfrontal lesions are impaired but due to differences in patients. As a result, medial frontal lesions are associated with poor performance.

The Mass-action Principle is one of two principles that Karl Lashley published in 1950, alongside the equipotentiality principle. The mass-action principle stated that the reduction in learning is proportional to the amount of tissue destroyed, and the more complex the learning task, the more disruptive the lesions are.[1] In other words, memory can not be localized to a cortical area, but is instead distributed through the cortex.

Lashley's research was an attempt to find the parts of the brain that were responsible for learning and memory traces, a hypothetical structure he called the engram. He trained rats to perform specific tasks (seeking a food reward), then lesioned varying portions of the rat cortex, either before or after the animals received the training depending upon the experiment. The amount of cortical tissue removed had specific effects on acquisition and retention of knowledge, but the location of the removed cortex had no effect on the rats' performance in the maze. This led Lashley to conclude that memories are not localized but widely distributed across the cortex.